In the recovery of oil from a subterranean formation, primary recovery methods utilizing the natural formation pressure to produce the oil typically allow recovery of only a portion of the oil contained within the formation. Additional oil and hydrocarbon compounds from the formation may be produced by improved or enhanced oil recovery (EOR) methods.
One enhanced oil recovery method utilizes a surfactant-polymer (“SP”) flood in an oil-bearing formation to increase the amount of oil recovered from the formation. An aqueous dispersion of a surfactant and a polymer is injected into an oil-bearing formation to increase recovery of oil from the formation, either after primary recovery or after a secondary recovery waterflood. The SP flood enhances recovery of oil from the formation by lowering interfacial tension between oil and water phases in the formation, thereby mobilizing the oil for production. In an SP flood, the polymer increases the viscosity of the SP fluid, typically to the same order of magnitude as the oil in the formation, so the mobilized oil may be forced through the formation for production by the SP flood.
In conventional SP oil recovery processes, the SP oil recovery formulation is typically designed to produce a low interfacial tension (IFT) between oil and water in the formation, often resulting in a stable microemulsion of oil and water in the formation. The microemulsion indicates very low IFT. The microemulsion may aid in mobilizing hydrocarbons for displacement through the formation and toward a production well for production and recovery from the formation.
In some cases, the SP oil recovery formulation may be designed to produce a low IFT between oil and water in the formation by selecting or tailoring one or more surfactants of the oil recovery formulation to induce the formation of a stable oil-water microemulsion at the salinity of the formation water. Although the microemulsion does not need to have a salinity matching formation water, the interfacial tension between formation oil and water in the presence of an SP oil recovery formulation is sensitive to the salt concentration of the aqueous SP dispersion and the salt concentration of water in the formation. An SP oil recovery formulation may be formulated by adapting the total salt concentration of the oil recovery formulation to match the total salt concentration of water in the oil-bearing formation into which the oil recovery formulation is to be introduced, and selecting or tailoring the one or more surfactants used in the oil recovery formulation to induce the formation of a stable oil-water microemulsion, indicative of a low oil/water interfacial tension. Alternatively, surfactant and polymer may be added directly to the produced water from the reservoir. The resulting SP oil recovery formulation is then injected into the oil-bearing formation to enhance oil recovery from the formation.
A limiting factor in using SP enhanced oil recovery is loss of surfactant from the oil recovery formulation to the formation. A portion of the surfactant of the SP oil recovery formulation may be adsorbed from the oil recovery formulation by the formation minerals and clays upon injection into the formation. Loss of surfactant to the formation decreases the surfactant concentration in the SP oil recovery formulation. While the total dissolved solids (TDS) concentration remains substantially unchanged as surfactant is lost to the formation, the ratio of total dissolved solids to surfactant increases and causes the surfactant to be relatively more soluble in oil than brine, referred to as “going over-optimum” (which has a high IFT between oil and water). In addition, higher levels of divalent cations in the SP oil recovery formulation compound this effect. Using an “over-optimum” SP oil recovery formulation is known to result in lower oil recovery. See Richard C. Nelson, The Salinity-Requirement Diagram—A Useful Tool in Chemical Flooding Research and Development, 22 SOC. OF PET. ENG. J. at 259-270 (1982). Lowering the salinity of the SP oil recovery formulation during the latter part of chemical injection may allow the SP oil recovery formulation to mix with the preceding injection fluid and stay in an “optimum” microemulsion region for a longer period of time during the flood. However, in off-shore operations, seawater is typically the primary, and often only, source of brine for the SP oil recovery formulation. As such, there may be limited ability to adjust the salinity of the brine without using expensive and/or inefficient filtering processes.
Improvements to existing SP enhanced oil recovery processes and compositions are desirable. In particular, processes and compositions effective to enhance oil recovery from an oil-bearing formation utilizing an SP oil recovery formulation that can keep the SP oil recovery formulation in an optimum microemulsion (low IFT) region or bring the SP oil recovery formulation back into an optimum microemulsion region without needing to change the total dissolved solids of the brine component are desirable.